Hydrocolloids made from naturally occurring gums are used extensively in the food, pharmaceutical and cosmetics industries. Sols of most such hydrocolloids are opaque or translucent. When such hydrocolloids are clarified, the cost is usually uneconomical or there is inevitably a loss in the physical properties of the hydrocolloids compared to the unclarified colloids. This can, for example, include substantial loss in viscosity. Examples of naturally occurring gums used in making hydrocolloid sols are konjac, guar, locust bean and xanthan.
Konjac Glucomannan:
Konjac glucomannan, the first word sometimes spelled “konjak”, is an acetylated glucomannan obtained from the tubers of the tropical plant, Amorphophallus konjac, commonly called “Devil's Tongue” because of its high content of oxalic acid. The konjac tuber is harvested following two or three year's growth, after which it has a diameter of 4-6″. Processing steps include slicing, placing the slices on racks, sun or open fire drying, pulverization, dry or wet milling to remove the oxalic acid and some of the starch content which adheres to the konjac sacs, followed by sifting or air classification. These oval sacs are about 2 mm long and are composed mostly of konjac glucomannan encased in a proteinaceous membrane. Starch granules adhere to the membrane and much of these can be removed by a 30% alcohol (aq) wash. Native konjac glucomannan has a wide variation of acetyl content since it is both a storage and a structural polysaccharide. The more acetylated forms of the konjac glucomannan are water-soluble and the more deacetylated forms are water-insoluble. This is a simplistic statement, however, since a whole spectrum exists with respect to degree of acetylation with some of the soluble species on the edge of insolubility and minor changes in environment, such as salt concentration, excessive heating, removal of protective hydrocolloids or other molecules, etc., can lead to insolubilization.
Crude konjac flour, the most common commercial form, is a well-known foodstuff in China and Japan and has recently gained FDA approval in the U.S. as a fat replacer in meat. This application is based on the fact that when konjac glucomannan is heated with alkali, about pH=˜7.5-11, deacetylation occurs and the resulting gel product is water insoluble and thermostable. The deacetylated gel or paste, commonly called “konnyaku” can even be fried at temperatures around 400° F. without melting or decomposing. If the gel formed by deacetylation is frozen and thawed, a tough, coherent spongeous mass is formed. Deacetylated konjac-containing films, foams, beads, and other forms can be prepared.
Konjac reacts with borate ion at alkaline pH to form amorphous gels as well as reacting synergistically with xanthan to form elastic gels.
As expected, there are numerous impurities in the crude, unclarified konjac. These include insoluble starches, cellulose, and nitrogen-containing impurities including proteins, many of which are derived from the konjac sac membrane. While crude konjac flours have numerous applications, as foods, as a soluble fiber source, as a fat replacement in meats, etc., the clarified form is preferable and in some applications, essential, for such applications as clear dessert gels, as a viscosifier or thickening agent for clear fluids, as clear capsules, films that are free from particulates, clear cosmetics (lotions and possibly gels in combination with clarified xanthan or borate), etc.
Guar Gum (Galactomannan):
Guar gum is a galactomannan polysaccharide obtained from the seed of the legume Cyanopsis tetragonolobus, an annual plant that grows mainly in and and semiarid regions of India and Pakistan. Guar is grown principally as a food crop for animals and as an ingredient in human foods and pharmaceuticals. The guar galactomannan is the major component in the seed endosperm, while the germ portion is mainly protein. In its commercial form, guar gum contains a significant number of impurities, including husks and other cellular debris, with the guar galactomannan comprising only about one-third of the product.
The guar galactomannan is composed of a backbone of (1→4)-linked β-D-mannopyranosyl units with single α-D-galactopyranosyl units connected by (1→6) linkages, with the ratio of galactose to mannose being about 0.55. There are many galactomannans in nature, each varying in this ratio which determines physical and chemical characteristics. Guar galactomannan is soluble in water to form viscous solutions. The actual viscosity values depend upon both the molecular weight and the purity. Guar gum imparts viscosity even in high ionic strength environments. Like konjac and locust bean gum, guar reacts synergistically with xanthan to form very viscous sols and/or gels, depending on proportions and concentrations. It also reacts with alkaline borate to yield amorphous gels.
Guar has numerous applications, some of which have been supplanted by guar derivatives. These range from oil drilling products to textile printing and dyeing to foods, cosmetics and pharmaceuticals.
Locust Bean Gum (Galactomannan):
Locust bean, carob, gum is a galactomannan polysaccharide obtained from the evergreen leguminous tree, Ceretonia siliqua L., which grows extensively in Spain is also cultivated in Italy, Cyprus and other Mediterranean countries. Locust bean gum is the refined endosperm of the seed and in its commercial forms locust bean gum contains a significant number of impurities, such as husk residue and cellular debris, depending on the grade.
Locust bean gum, like guar is a galactomannan having the same basic structure. However, there are considerably fewer galactose side-chains in the locust bean galactomannan. The galactose to mannose ratio is 0.25, compared with guar's 0.55. This lower degree of branching is responsible for differences in properties, especially solubility. While guar is mostly soluble in cold water, locust bean gum is not. Dispersions must be heated to about 85° C. to achieve full viscosity. Weak gels are formed when hot sols of locust bean gum are allowed to cool quiescently. Locust bean gum will gel in the presence of borate ion at alkaline pH. It will react synergistically with xanthan to form a gel and will impart elasticity to agar and κ-carrageenan. Locust bean gum is stable over a wide range of pH values, but is rapidly degraded by enzymes found in indigenous microbes.
While guar and guar derivatives have replaced locust bean gum in a number of applications because of cost-effectiveness considerations, locust bean gum is still used in dairy and frozen dessert applications, meat products, pet foods, and the textile industry.
Aloe Acemannan:
Aloe acemannan is a mannan first isolated from Aloe barbadensis (var. Miller) by McAnally at Carrington Laboratories and is pharmacologically active. In its commercial state, it contains fine water-insoluble particulates that impart turbidity to the sol. About 80% of the commercial product is a polysaccharide that is composed of a mannose backbone of from 5-50,000 linked units, with >75% being greater than 10,000. Commercial acemannan is partially water soluble and forms viscous sols. It, too, reacts synergistically with xanthan to form elastic gels and alkaline borate to form amorphous gels.
Xanthan Gum:
Xanthan gum is a so-called heteropolysaccharide obtained from the fermentation of Xanthamonas campestris. The polymer backbone is composed of (1→4)-linked β-D-glucopyranosyl units, the same as cellulose. Trisaccharide side chains are attached to alternate D-glucosyl units. These are composed of acetyl mannose, glucuronic acid, and mannose residues, with about half of the terminal mannose units containing pyruvate as a 4, 6 cyclic acetal. Many commercial xanthan gum products form somewhat turbid sols, although most of the cellular debris is removed by centrifugation as a processing step. A few higher-value commercial products form an essentially clear sol as a result of an additional filtration step in the processing.
Xanthan gum imparts high viscosity to aqueous solutions at low concentrations. It is compatible with a wide pH range (1-13), being quite stable at ambient temperature for all values. Xanthan gum sols will also add viscosity to solutions having high salt content. Xanthan interacts synergistically with galactomannans, such as guar gum and locust bean gum, and konjac glucomannan to significantly increase viscosity and/or form gels. With these unique properties and its GRAS listing as a food additive, xanthan gum has a wide range of applications, from oil well drilling to salad dressings, cosmetics, and pharmaceuticals.
Clarified Hydrocolloid Composites:
Hydrocolloid composites with varying components in varying weight/weight rations can be prepared by combining their sols and then recovering the product by one of any number of available methods. Although co-processed hydrocolloids and dry physical mixtures of hydrocolloids powders exhibit essentially the same solution properties, dispersion and water absorption properties can be significantly different and vary according to the relative proportions.
Clarified Hydrocolloid/Borate Interaction Products:
At pH values between about 7.5 and 9.0, the borate ion will interact with polymers containing cis-1,2-diols to form more viscous, amorphous systems. These polymeric diols can be synthetic, semi-synthetic, or natural. Some of the more common polymers which undergo this reaction are the polyvinyl alcohols; galactomannans, such as guar gum and locust bean gum; and glucomannans, such as konjac and Aloe (ace) mannans. Depending on the concentration of the polymer, or polymers if two or more are used, the borate, and other additives, if any, the consistency can vary from somewhat viscous fluids to crisp amorphous solids. At selected concentrations of the individual components, the reaction products behave like “healable” solids that will flow at body temperatures. Other soluble and insoluble materials can be added to impart desired properties, such as increased fluid absorption, fluid donation, elasticity, etc.